Injection of hydrated lime and other alkaline materials is a promising technology for control of acid, such as NOX (NO and NO2), HCl, and SOX (SO2 and SO3) from coal- and biomass-fired sources. Acid gas control is becoming obligatory due to the problems arising from increased corrosion, acid mist emissions and associated impacts to plant opacity, and the propensity of certain acid gases, such as SO3, to interfere with powdered activated carbon (“PAC”) used for mercury capture from these sources. Concerns with SO3 emissions have increased due to selective catalytic reduction reactors (“SCRs”) oxidizing sulfur dioxide to sulfur trioxide. SCRs are being installed on an increasing number of coal-fired sources for control of nitrogen oxides. SOx species are also present in flue gas at elevated levels when burning high sulfur coals. The presence of sulfur species and the ammonia reactants used for nitrogen oxide control can combine to form condensable compounds that foul or degrade air heater performance over time.
Injection of dry alkaline sorbents to control acid gas emissions continue to be used successfully at many coal-fired sources to chemically control emissions. When dry alkaline materials are injected into a gas stream for the purpose of controlling acid gases, the desired chemical reactions occur in the flue gas stream.
A major goal of any injection system is to maximize the desired reactions and minimize undesired reactions and/or interactions with the walls and mechanical systems downstream. As an example, one of the desired acid gas reactions between hydrated lime and SO3 is shown below:Ca(OH)2+SO3→CaSO4+H2Ohydrated lime+sulfur trioxide=calcium sulfate+waterOne of the major undesired reactions that occurs within the alkaline sorbent injection system is chemisorption of carbon dioxide by hydrated lime with carbon dioxide:Ca(OH)2+CO2→CaCO3+H2Ohydrated lime+carbon dioxide=calcium carbonate+waterThe rate of carbonate formation is believed to be a temperature-dependent process; that is, the higher the gas temperature the greater the rate of conversion of hydrate to carbonate. Calcium carbonate has been shown to be inversely soluble, therefore increasing temperature leads to greater carbonate deposition within the injection system. Managing the temperature of the injection system and lime carrier gas reduces the rate of formation and subsequently minimizes the deposition of calcium carbonate as a precipitate or scale. Due to the presence of air in the injection system, CO2 is always available for reaction in either the carrier gas or in the flue gas that contaminates the carrier gas through leakage or recirculation into the injection system. Therefore, thermal management of the injection system is employed to successfully moderate carbonate formation, reduce lime consumption, and increase system reliability.
A carrier gas treatment system 200 according to the prior art includes an optional dehumidifier 204 to reduce moisture levels (adsorption of moisture can result in caking, agglomeration and/or deposition of the sorbent within the feed, conveying, and injection components), a regenerative or positive displacement blower 208 (air having a pressure in the range of 3 to 25 psi), and a refrigerated air dryer 212 and/or after cooler 216. Other configurations and arrangements of the illustrated equipment are possible. This system 200 generally reduces the dew point of the conveying air to a temperature just above 32° F. Although the reaction of calcium hydroxide and carbon dioxide to air is slowed by the gas dehumidification and cooling, the degree of dehumidification and cooling is limited. Accordingly, conventional systems 200 generally experience debilitating issues with scaling, abrasion, plugging in the lines, lances, and other conveying surfaces in the system 200.
One prior approach is depicted in FIG. 17. The system of FIG. 17 uses a positive or regenerative blower drawing in filtered ambient air. The entire air discharged from the blower is applied to the cone of the silo to fluidize the additive material (which is hydrated lime) and enhance the material flow properties from the silo by aeration. Since hydrated lime is a natural CO2 sorbent, the hydrated material in the silo removes a percentage of the CO2 from the applied air as it bubbles through the silo. This method also serves to remove moisture from the applied air since Ca(OH)2 is a hygroscopic material. The lean CO2 and H2O air stream is then pulled from the head space of the silo by a fan at the exit of a dust collector. The exhaust from the dust collector fan is then used to supply the motive air supply blower and convey material from the feeder at the bottom of the silo cone. The resulting conveyance air has lowered CO2 and moisture content.
While this system demonstrated significant reliability and pneumatic handling improvements, it can have limitations. Successful acid gas mitigation requires that the inherently high reactivity of hydrated lime be preserved. By first using the hydrated lime as a CO2 sorbent, this system converts expensive highly reactive hydrated lime to calcium carbonate and delivers the carbonate form mixed with degraded hydrated lime. The system introduces water to the stored sorbent in the silo originating from the carbonate reaction. Water can agglomerate the finely sized calcium hydroxide particles and ultimately impact the ability of this material to be subsequently metered from the silo and conveyed to and effectively dispersed within the duct.
The system has failed in operation. The time-to-failure can be described as the time required for the carbonate scale to form and occlude the system to the extent that material can no longer be conveyed, at which point a labor-intensive mechanical cleaning of the conveyance system internal surfaces or component replacement is required. Another limitation of the the system is that the level of CO2 removed varies with the material level in the silo. Because the silo level changes over time, the delivered motive air quality will vary. In other words, the silo level is directly proportional to the contact time between the air and sorbent. Thus, lower silo levels result in lower contact times, thereby introducing variability to the CO2 concentration in the carrier gas.
These problems increase the necessity for time-consuming and expensive manual cleaning and maintenance of the introduction system due to precipitate and scale formation.